[quote name='Setherial']I myself haven't used it but I purchased it at a GoodWill so I'm guessing it has been used. I believe the CD-Key is only used to "Install" the game on your system however I may be wrong.[/QUOTE]
I suggest you de-list the item, then. Once the cd-key is used it is tied to the Steam account it was installed on, rendering the game useless to anyone else.